c++ - 是否可以使 std::string 始终包含小写字符串？

Question

是否可以使 std::string 始终包含小写字符串？下面是我将如何使用它:

typedef std::basic_string<...> lowercase_string;

void myfunc()
{
  lowercase_string s = "Hello World"; // notice mixed case
  printf(s.c_str());                  // prints "hello world" in lowercase
  std::string s2 = s;
  printf(s2.c_str());                 // prints "hello world" in lowercase
}

Answer 1

您可以编写自己的字符特征并将其作为第二个模板参数传递给 std::basic_string。

这是一个最小示例:

template<typename T>
struct lowercase_char_traits : std::char_traits<T>
{
    static T* copy(T* dest, const T* src, std::size_t count )
    {
         for(size_t i = 0 ; i < count ; ++i)
              dest[i] = std::tolower(src[i]);
         return dest;
    }
    static void assign(T & out, T in)
    {
       out = std::tolower(in);
    }
    //implement other overload of assign yourself

    //note that you may have to implement other functionality 
    //depending on your requirement
};

然后将 typedef 定义为:

typedef std::basic_string<char, lowercase_char_traits<char>> lowercase;

这是一个测试程序:

int main() 
{
    lowercase s1 = "Hello World";
    std::cout << s1.c_str() << std::endl;

    lowercase s2 = "HELLO WORLD";
    std::cout << std::boolalpha << (s1 == s2) << std::endl;

    lowercase s3 = "HELLO";
    s3 += " WorL";
    s3.append("D");
    std::cout << std::boolalpha << (s1 == s3) << std::endl;

    std::cout << s2.c_str() << std::endl;
    std::cout << s3.c_str() << std::endl;
}

输出:

hello world
true
true
hello world
hello world

很酷，不是吗？

• Online demo

---

请注意，要拥有一个完全可用的小写字符串类，您可能还需要定义 lowercase_char_traits 的其他功能，具体取决于您希望此类的行为.

有关详细信息和解释，请查看 Herb Sutter 的精彩文章:

• So you want a case-insensitive string class? Your mission, should you choose to accept it, is to write one.

希望对您有所帮助。